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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 2, JANUARY 15, 2011
Design and Fabrication of Tb3+-Doped Silicon
Oxy-Nitride Microdisk for Biosensor Applications
Hoon Jeong, Shinyoung Lee, Gun Yong Sung, and Jung H. Shin
Abstract—We report on fabricating Tb-doped SiON microdisks
on a Si chip for low-cost all-Si biosensor applications. The
120-nm-thin 10- m diameter pedestal-type microdisks were
designed and fabricated for optimum biosensing capabilities.
Whispering gallery modes of fabricated microdisks were measured via UV top pumping and side-photoluminescence (PL)
measurement, and Q-factors of several hundreds were obtained.
Using the microdisk resonators to sense streptavidin using biotin
functionalization is also demonstrated.
Index Terms—Biosensors, microresonators, rare-earth doping.
I. INTRODUCTION
N recent years, planar microresonators have attracted a
particular attention as a key component for integrated,
‘lab-on-a-chip’ biosensors [1], [2]. Adsorption of biomaterial
changes the effective refractive index of the resonator and
thus its resonance positions, which then can be used as an
indicator of the amount of the adsorbed biomaterial [3]. With a
sufficiently high quality factor (Q), even single molecule level
detection has been demonstrated [4].
In most cases, resonance positions are obtained by measuring
the transmission of a signal from an external source [1], [2],
[5]–[7]. Unfortunately, doing so generally requires an expensive tunable laser that can scan over a wide wavelength range,
which can increase the cost of the sensing apparatus and make a
monolithic integration of such biosensors on a Si chip difficult.
A simpler approach is to use resonators made of luminescent
material to enable a direct observation of resonance peaks in
the photoluminescence (PL) spectra [8]. In this letter, we report on design and fabrication of Tb doped silicon oxy-nitride
(SiON) microdisk resonators on a Si chip for biosensor applications. This combination of Tb and SiON was chosen as Tb
ions show efficient green emission at 541 nm
as the
state is the first excited 4f state of Tb [9]. Thus,
doping SiON with Tb allows us to obtain a Si-based material
I
Manuscript received July 04, 2010; revised October 20, 2010; accepted October 30, 2010. Date of publication November 09, 2010; date of current version
December 30, 2010. This work was supported in part by MEST (R31-2008000-10071-0, 20100029255), in part by OPERA, and in part by the Top Brand
R&D program of MKE (09ZC1110: Basic Research for the Ubiquitous Lifecare
Module Development).
H. Jeong and S. Lee are with the Department of Physics, KAIST, Daejeon
305-701, Korea.
G. Y. Sung is with Biosensor Research Team, ETRI, Daejeon 305-700, Korea.
J. H. Shin is with the Department of Physics, KAIST, Daejeon 305-701,
Korea, and also with the Graduate School of Nanoscience and Technology
(WCU), KAIST, Daejeon 305-701, Korea (e-mail: [email protected]).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2010.2091268
that emits light in the green range, where the detection efficiency
of Si-based) detectors is high, enabling possible monolithic integration, and where the absorption by water is at the lowest, for
possible use in a realistic biological environment. Furthermore,
Tb ions are sensitized by the host SiON [10], and can be excited via top-pumping in the Ultraviolet (UV) range where SiON
absorption is strong, and emit in the green range where SiON absorption is weak, allowing us to obtain high excitation efficiency
without self-absorption. Finally, SiON has a relatively high refractive index of 1.8, enabling fabrication of compact resonators
with large free spectral range such that simple diffractive optics
can be used to observe resonance peaks. Based on finite-difference time domain (FDTD) simulations, we identify microdisks
m diameter as being opwith 100 nm thickness and
timum for biosensing. The fabricated disks have Q-factors in
the 500 range, and sensing of streptavidin using biotin surface
functionalization is demonstrated.
II. EXPERIMENT
A. Resonator Design
Performance of biosensors is characterized by sensitivity
,
defined as the change in the resonant wavelength per change in
refractive index unit (RIU) of the surroundings, and the detec, defined as
, where
is the
tion limit
resolution limit. For high sensitivity, a large overlap between
the optical mode and the surroundings is required. On the other
hand, for low detection limit, a high Q factor is desired, which
requires that the mode be well confined within the resonator.
Thus, FDTD simulations were used to first evaluate and design
microdisks for optimum biosensing performance.
Fig. 1(a) shows that the air-mode overlap (the fraction of energy in the surrounding air) of transverse-electric (TE) whispering gallery modes (WGM) increases monotonically with decreasing disk thickness, while the radiation-limited Q-factor of
the modes near the Tb luminescence peak of 541 nm deto
as the disk thickness is
creases suddenly from
reduced to below 100 nm. As a result, we find that a disk thickat
RIU.
ness of 100 nm gives the lowest value of
FDTD simulations also indicate that a 100 nm thick disk is
too thin to support transverse-magnetic (TM) modes, in agreement with previous reports [11], which significantly simplifies
the analysis (data not shown). We note, however, that the actual
and therefore
are more likely to be determined by the
system resolution, as will be shown later.
A similar calculation on the effect of varying the diameter of a
120 nm thick microdisk resonator is shown in Figs. 2(a) and (b).
We find that the mode overlap and sensitivity increase with decreasing diameter, while the Q-factor drops strongly if the di-
1041-1135/$26.00 © 2010 IEEE
JEONG et al.: DESIGN AND FABRICATION OF Tb
-DOPED SILICON OXY-NITRIDE MICRODISK
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Fig. 1. (a) Calculated effect of disk thickness on the Q-factor and mode overlap.
(b) Calculated effect of disk thickness on S and DL.
Fig. 4. (a) Side-PL obtained from a single disk, showing sharp WGM resPL.
onance peaks at expected positions superimposed on background Tb
Shown in the inset is a CCD image of the disk seen from the side. (b) Resonance peaks obtained by background subtraction, together with Lorentzian fits.
Obtained Q-factors are indicated.
Fig. 2. (a) Calculated effect of disk diameter on the Q-factor and mode overlap.
(b) Calculated effect of disk thickness on S and DL.
Fig. 3. SEM image of fabricated SiON:Tb microdisk resonator, together with
calculated E -field distribution of a first-order TE whispering gallery mode.
ameter is decreased to below 10 m such that a disk diameter
m gives the lowest value of
at
RIU.
of
B. Resonator Fabrication and Characterization
Based on the above results, 120 nm-thin, Tb-doped SiON film
was deposited on a Si substrate [10]. 10 m diameter microdisks
were then fabricated using e-beam lithography and dry etching
using CHF and O . Finally, selective etching of the Si substrate
with 30% KOH solution was used to selectively etch the Si substrate, and create a narrow pedestal for the SiON disk. Fig. 3
shows a typical scanning electron microscope (SEM) image of
a fabricated disk. Also shown is the calculated E distribution
of a first-order TE WGM that confirms that the optical mode is
m of the disk edge and well away from
confined to within
the pedestal region.
The resonance spectra were obtained using a side-PL setup
[11]. 325 nm light from a HeCd laser was focused onto a single
microdisk. Microscope objectives were used to collect the signal
emanating from the microdisk edge and focus it onto the entrance slit of a monochromator. The excitation power was about
10 mW, and the spectral resolution of the optical setup was
about 0.5 nm. Fig. 4(a) shows the side-PL spectra from a microdisk, together with the calculated WGM resonance peak positions for the first-order TE modes. Shown in the inset is a
charge-coupled device (CCD) image of the disk seen from the
side, showing the green Tb luminescence visible to the naked
eye. We can clearly observe the resonance peaks, superimposed
on the background Tb PL peak, at expected positions. Also
shown for comparison is a typical background Tb PL spectrum from an unpatterned SiON:Tb film, normalized to overlay
the background PL. By subtracting the background Tb PL
spectrum, we can obtain the resonance peak spectrum, as is
shown in Fig. 4(b). The Q-factors of the WGM resonances, as
determined by Lorentzian fitting, are in the range of 200–500.
These values are much lower than the calculated values, indicating substantial external loss through mechanisms such as
edge scattering [11], [16]. We also note that the edge-emission
from a planar cavity can modify the background spectrum shape
[12], [13]. However, given the narrow width of the Tb peak,
and the good agreement between the calculated and observed
resonance peaks, we shall neglect such effect in the analysis.
C. Demonstration of Bio-Sensing
Bio-sensing capabilities of the microdisks were investigated
using the biotin-streptavidin binding system. First, microdisks
were cleaned and oxidized by a 10 min immersion in a 80 C,
1:1:6 mixture of H O , HCl, and DI water. Next, the microdisks
were silanized with a 10% solution of APTES, and rinsed. Finally, the sample was immersed in a solution of NHS-LC-LCBiotin for 12 hours at room temperature to functionalize the surface with biotin. For sensing, the biotinylated disks were reacted
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 2, JANUARY 15, 2011
25,000 [16]. As the radiation-limited Q-factor of 10 m SiON
, we believe that with better controls over fabdisk exceeds
rication and signal collection that can utilize the unique advantage of top-pumped green luminescence from a Si-based thin
film, Tb-doped SiON can be used as a new material basis for a
low-cost, compact, integrated compact bio-sensors.
III. CONCLUSION
Fig. 5. (a) Background-subtracted side-PL from the same microdisk after each
steps in bio-functionalization. Note the redshift after each step, as indicated
by the arrows. (b) Side-PL obtained after same processing but with solutions
without any biomaterials.
with streptavidin dissolved in pH 7.4 sodium phosphate buffer
solution (PBS) at a concentration of 1 mg/1 ml, and rinsed.
Fig. 5(a) shows the effect of bio-film functionalization and
subsequent biotin-streptavidin reaction on the positions of
WGM resonance peaks, obtained from the same disk after each
steps. All measurements were obtained after rinsing and drying
the samples in the air. We find that the peaks red-shift upon biotinylation and streptavidin reaction by about 3 nm but without
any changes in the width and hence Q-factors. However, as
Fig. 5(b) shows, a disk that underwent identical processes but
with solutions that did not contain any biomaterial does not
show any redshift. In fact, we observe a slight blueshift of
1 nm, which we attribute to slight etching of the disk by the
buffer solution.
D. Discussion
The fact that we can observe WGM peaks at all throughout
the process without any changes in the Q-factors demonstrates
that the microdisks, despite their thickness of only 120 nm, are
robust enough to withstand the aggressive cleaning, immersion,
and subsequent drying cycles, and thus are able to fulfill their
purpose, which was to enable direct observation of the resonance positions using simple spectroscopy rather than an expensive external laser and a delicate coupling setup. Also, the
redshift in the positions of WGM peaks is consistent with the
expected increase in the effective size of the microdisk due to
adsorption of the biotin and streptavidin. This, together with the
lack of such redshift when they are not present, demonstrates
that the shift in the peak positions reflects the bio-sensing capability of the Tb doped SiON microdisks in the top-pumping,
side-PL scheme and not a process-related instability. Finally,
based on the FDTD simulation of the WGM resonances, and assuming a refractive index of 1.38 for the biomaterials [14], we
estimate the thickness of the biomaterial layers to be 11.5 nm,
corresponding to a simulated sensitivity of 0.26 nm/nm for surface adsorption of bio-film.
The sensitivity of 0.26 nm/nm is comparable to other planar
microresonator based biosensors [15]. However, the limit of detection for the Tb-doped SiON microdisk resonators are currently limited by the low Q-factor and the low system resolution of 0.5 nm. Previously, however, we have reported on fabricating SiNx microdisk resonators with an intrinsic Q-factor of
In conclusion, we have designed and fabricated 120 nm thick,
10 m diameter Tb-doped SiON microdisk resonators for biosensing applications. WGM resonance peaks in the green region
were easily excited using UV top-pumping, and detected with
side-PL setup. Using biotin-streptavidin reaction, bio-sensing
capabilities of the microdisks are demonstrated.
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